Preventing Metal Shunting in Perovskite Solar Cells during Reverse Bias

Perovskite solar cells (PSCs) are emerging as promising materials for photovoltaic energy conversion due to their rapidly increasing power conversion efficiencies and cost-effective fabrication procedures. Many research efforts are currently dedicated to investigating external stressors that perovskite solar cells experience in the field – oxygen, elevated temperatures, and moisture/humidity. However, there remains a knowledge gap concerning the behavior of this technology when subjected to another real-world external stressor for solar panels – reverse bias.<br/><br/>PSCs typically exhibit low breakdown voltages in reverse bias (&lt;5V), making the integration of traditional bypass diodes used in Si arrays difficult due to the increased production costs^{[1] [2]}. Therefore, numerous efforts have been made to identify and address the root causes of reverse bias-induced degradation of PSCs. These breakdown mechanisms of PSCs have been directly correlated with voltage-induced ion migration and the formation of hotspots when using metal electrodes ^{[2] [3]}. The diffusion of metallic species in PSCs has been recorded during regular operation, but is accelerated under reverse bias conditions as elevated electric fields facilitate ion migration and initiate interactions between mobile ions and metal species at the contacts.<br/><br/>We have studied a wide variety of strategically chosen perovskite solar cells under reverse bias, and we observe the formation of these metallic shunts in PIN architectures with metal electrodes. We employed metals with various electrochemical reactivities (Ag, Au, Cu, & Al) and found that metal shunting is most prominent in silver metal electrodes, one of the most popular contact materials used in perovskite solar cells. Metal diffusion/shunting most-often results in catastrophic failure of the device when returned to normal operating conditions. We attribute this metal shunting behavior to the preferred oxidation state of the metals and the increased activation energy for these metal ions to diffuse through the perovskite device architecture. We show that metal shunting/diffusion in our inverted perovskites can be prevented by simply using higher oxidation-state metals (Au, Cu, & Al). We also investigate the application of interlayer additives, including barrier layers deposited via atomic layer deposition (ALD), chromium (Cr), benzotriazole (BTA), and other chelating agents commonly employed in water treatment facilities to address metal accumulation.<br/>[1] E. J. Wolf, "Designing Modules to Prevent Reverse Bias Degradation in Perovskite Solar Cells when Partial Shading Occurs," <i>Solar RRL, </i>2021.<br/>[2] C. Wang, "Perovskite Solar Cells in the Shadow: Understanding the Mechanism of Reverse-Bias Behavior toward Suppressed Reverse-Bias Breakdown and Reverse-Bias Induced Degradation," <i>Advanced Energy Materials , </i>vol. 13, no. 9, 2023.<br/>[3] A. Bowring, "Reverse Bias Behavior of Halide Perovskite Solar Cells," <i>Advanced Energy Materials, </i>vol. 8, no. 8, 2017.